Electron radiogram developer with image field screen

ABSTRACT

Apparatus for developing electrostatic images on a receptor sheet by attracting charged toner powder particles to the sheet for subsequent fixing in place. A reduction in the amount of background toner in the visual image is achieved by positioning an image field screen in front of the receptor sheet and maintaining a potential between the screen and the sheet such that the toner is deposited on the sheet only at localities having a charge potential greater than that of the screen.

United States Patent Larson et al.

ELECTRON RADIOGRAM DEVELOPEIT WITH IMAGE FIELD SCREEN Inventors: Ray W. Larson, Santa Monica;

Arthur Lee Morsell, Tarzana; Eric P. Muntz, Pasadena; Murray S.

Welkowsky, Los Angeles, all of Calif.

Assignee: Xonics, Inc., Van Nuys, Calif.

Filed: Aug. 24, 1972 Appl. No.: 283,311

US. Cl 118/638, 117/17.5, 118/637 Int. Cl G03g 13/00 Field of Search 118/629, 637, 638;

References Cited UNITED STATES PATENTS 11/1959 Rosenthal 222/146 12/1966 St. John 101/114 1 Oct. 22,1974

3,295,440 1/1967 Rarey et a1, 101/114 3,357,403 12/1967 Donalies 118/637 3,396,700 8/1968 Donalies 1 18/637 3,504,624 4/1970 Kennedy, Jr 101/114 Primary ExaminerRobert R. Mackey Assistant ExaminerLeo Millstein Attorney, Agent, or FirmHarris, Kern, Wallen & Tinsley [57] ABSTRACT Apparatus for developing electrostatic images on a receptor sheet by attracting charged toner powder parti-.

cles to the sheet for subsequent fixing in place. A reduction in the amount of background toner in the visual image is achieved by positioning an image field screen in front of the receptor sheet and maintaining a potential between the screen and the sheet such that the toner is deposited on the sheet only at localities having a charge potential greater than that of the screen.

4 Claims, 2 Drawing Figures ELECTRON RADIOGRAM DEVELOPER WITH IMAGE FIELD SCREEN This invention relates to powder cloud developing of electrostatic images such as occurs in xerography and ionography. In a typical system, an electrostatic image is produced on a receptor sheet, such as a sheet of selenium or a sheet of a plastic dielectric. Toner powder particles are deposited on the receptor sheet, with the toner particle density being a function of the electrostatic charge on the receptor. The toner is fixed in place, as by heating.

Conventional developing chambers are described in US. Pat. Nos. 3,646,910 and 3,648,901 and in the art cited therein.

In the development of any electrostatic image, image quality is deteriorated whenever toner is deposited without regard to the charge present on the image carrying receptor. This background toner decreases the contrast of the visual image. The contrast degradation due to presence of background toner is particularly evident in systems where the electrostatic charge on the receptor is small, such as in electronradiograms, such as those produced by X-ray ionography. Ideally, the toner powder should be deposited in proportion to the amount of surface charge on the receptor, with the areas having zero charge receiving no toner. Further, where there is a background charge throughout the receptor sheet, such as is produced by scattered radiation in X-ray systems, visual image contrast is improved by depositing toner particles only where the local charges are greater than the generally uniform background charge.

The present application is directed to an apparatus for eliminating or reducing the background toner powder in the development of electrostatic images. In summary, the invention provides an apertured electrode plate or screen adjacent the receptor carrying the electrostatic image, with the screen at a potential related to that of the background of the receptor so that charged toner particles are attracted through the screen to the receptor only at localities having a charge above that of the background.

Accordingly, it is an object of the invention to provide such a new and improved apparatus utilizing an image field screen in a powder cloud developer for improved visual image quality with increased contrast. Other objects, advantages, features and results will more fully appear in the course of the following description. The drawing merely shows and the description merely describes preferred embodiments of the present invention which are given by way of illustration or example.

In the drawing:

FIG. 1 is a vertical sectional view through a powder cloud developer incorporating the presently preferred embodiment of the invention; and

FIG. 2 is a vertical sectional view of the chamber of FIG. 1 taken at ninety degrees to the section of FIG. 1.

The apparatus includes a chamber or housing 10, with a backing electrode 11 positioned at an opening in the top 12 of the chamber. A receptor sheet or plate 13 having the electrostatic charge image thereon is carried on the backing electrode 11. The receptor sheet may be a conventional element such as a plastic sheet or a selenium sheet with the electrostatic image produced by conventional techniques such as xerography or ionography.

An apertured plate, preferably a wire screen 15 is carried in the chamber 10 adjacent the receptor sheet, as by resting on chamber wall brackets 16. The screen 15 is positioned generally parallel to the receptor sheet 13, with the spacing between sheet and screen determined by the potential due to the charge density on the receptor sheet. Typically, the spacing is in the range of 0.25 mm to 10 mm. With a low charge density such as is often obtained in ionography, the spacing may be in the order of a few millimeters.

Means are provided for moving the screen 15 relative to the image on the sheet 13 in a reciprocating or oscillating manner so that an image of the screen itself is not formed on the finished picture. The motion of the screen relative to the sheet desirably should be uniform and linear during development. Conventional means may be provided for producing the movement, which preferably is in a sawtooth pattern in phase with shutter opening. A motor 20 may be coupled to the screen 15 via crank 21 and arm 22. The motor 20 and drive mechanism is illustrated outside the chamber 10 in FIG. 2, but may be positioned within the chamber if desired.

A cloud of charged toner powder particles is produced within the chamber by conventional means, such as the toner injection mechanism illustrated in FIG. 2 comprising control unit 24, air supply 25, valve 26, toner supply 27, and nozzle 28. A baffle 29 may be positioned within the chamber above the outlet of nozzle 28 for controlling flow out of the nozzle. After development, excess toner may be removed from the chamber by conventional means such as nozzle 32, valve 33, and vacuum exhaust unit 34.

A charge selection electrode 37 is mounted in the chamber 10 and electrically insulated from the chamber by standoff insulators 38.

A mechanism is provided for shielding the screen 15 and receptor sheet 13 from the powder cloud during toner injection and typically may comprise a focal plane shutter type mechanism. Shutter plates 40, 41 rest on wall brackets 42, 43, respectively. The shutter plates are moved between the closed position shown in phantom lines in FIG. 1, and the open position shown in solid lines in FIG. 1, by a drive mechanism which may be conventional and which is shown diagrammatically as a motor 45, crank 46 and arm 47. The drive mechanism for the shutter may be within or without the chamber 10 as desired.

Baffles 50 may be provided within the chamber for generally confining the cloud of toner particles to the zone between the selection electrode 37 and the screen 15.

The chamber 10 may be of metal and serve as circuit ground, with the backing electrode 11 forming a portion of the chamber and being at circuit ground. An electric field is produced within the chamber by connecting an electric power source 52 between the chamber and the charge selection electrode 37. A biasing potential may be provided for the screen 15 by connecting another electric power source 53 between the chamber and the screen, with the screen insulated from the chamber as by forming the brackets 16 and drive rod 22 of electrical insulating material. The polarities for the sources 52, 53 are dependent on the polarity of the charge on the receptor sheet. In the example illustrated, the electrostatic charges on the receptor sheet are negative and therefore the electrode 37 is made positive with respect to the chamber and backing electrode 11. The screen may also be operated at circuit ground potential with the potential source 53 omitted, but it is preferred for reasons to be discussed below to have a low potential on the screen, with the charge selection electrode-to-screen potential of opposite polarity to the screen-to-backing electrode potential. In the embodiment illustrated, the screen 15 is negative with respect to circuit ground.

In operation, the shutter is closed and a charged receptor sheet is placed in position as illustrated, with the charged side down. A cloud of toner particles is introduced into the chamber through the nozzle 28. The control 24 opens the valve 26 for a short period of time providing a pressurized burst of air to the toner unit 27. This conventional procedure provides the cloud of charged toner particles in the chamber, with some particles charged positive and some particles charged negative. The negative charges on the receptor sheet will attract only positive charged toner particles and the negative charged toner particles are attracted to the charge selection electrode 37.

The shutter is maintained at circuit ground potential and prevents direct bursts of toner powder to the screen and receptor sheet. Also, maintaining the shutter closed permits formation of a substantially uniform cloud of particles throughout the chamber and provides for damping of turbulence resulting from the pressurized air burst. In a typical chamber, the source 52 is selected to provide a field in the order of 500-l,000 volts per centimeter.

After the uniform cloud is produced, the shutter is moved to the open position exposing the screen 15 and receptor 13 to the positively charged toner particles. First consider operation with the potential of the source 53 zero. At localities where charges exist on the receptor 13, a corresponding electric field is established between the receptor and screen and toner particles are attracted through the screen to the receptor surface, in proportion to the strength of the field. Where no charge exists on the receptor, there is no field between receptor and screen and no force attracting toner particles to the receptor. With this arrangement, there is substantially no toner deposited on the receptor in the zero charge background areas, resulting in a visual image of improved contrast.

Scattered radiation is a problem in X-ray radiographic techniques since the scattered radiation may typically represent half of the X-rays reaching the imaging device. The scattered radiation is generally uniformly distributed over the image. A moving lead grid was developed for use with X-ray film systems to serve as a collimator for reducing scattered radiation. This device is usually referred to as a Potter-Bucky grid and is oscillated during exposure so that the shadows of the grid element do not appear on the image. While the oscillating grid is helpful in reducing the adverse effect of scattered radiation, it also absorbs a substantial portion of the unscattered radiation, requiring higher dosages for usable pictures. The moving grid arrangement may also be used in X-ray ionography for reducing scattered radiation which appears on the receptor as a generally uniform charge level over the image area.

It has been found that the visibility of the scattered radiation in the finished visual image can be substantially reduced by suitably biasing the screen 15 with respect to the receptor 13. The source 53 is used to provide the bias potential on the screen 15. With the screen 15 biased with respect to the circuit ground potential at the backing electrode 11 carrying the receptor sheet, charged toner particles are attracted through the screen 15 to the receptor 13 only at localities carrying a charge greater than a particular value. This minimum value of charge corresponds to the generally uniform charge produced across the receptor by the scattered radiation, so that the visual image is formed only by the additional charge resulting from unscattered radiation.

In operation, the minimum potential of the electrostatic image on the receptor may be determined by sampling the image surface at several locations with an electrometer. The magnitude of the biasing potential is then adjusted at the variable source 53. The preferred bias potential for a developing chamber will be a function of the amount of scattered radiation in a particular image and may readily be determined by experiment.

We claim:

I. In a powder cloud developer having a developing chamber, means for producing a cloud of charged toner powder particles in the chamber, and means for supporting in the chamber a receptor sheet having an electrostatic image, the improvement comprising in combination:

an apertured image field plate mounted in said chamber adjacent said receptor sheet;

a charge selection electrode mounted in said chamber, with said particle cloud produced between said plate and electrode;

means for producing an electric field between said plate and electrode for attracting particles of one polarity to said electrode and particles of the opposite polarity to said plate, with the charge on said sheet attracting said particles of opposite polarity through the plate apertures to said sheet;

a shutter mounted in said chamber between said plate and electrode and adjacent said plate; and

means for moving said shutter between a closed position blocking said cloud of particles from said plate and an open position permitting movement of particles to said plate.

2. A developer as defined in claim 1 including means for imparting oscillatory motion to said apertured plate in a plane substantially parallel to said receptor sheet.

3. A developer as defined in claim 2 wherein said apertured image field plate is a wire screen.

4. A developer as defined in claim 3 wherein said shutter comprises first and second planar elements positioned adjacent each other when in said first position and moved away from each other when in said second position. 

1. In a powder cloud developer having a developing chamber, means for producing a cloud of charged toner powder particles in the chamber, and means for supporting in the chamber a receptor sheet having an electrostatic image, the improvement comprising in combination: an apertured image field plate mounted in said chamber adjacent said receptor sheet; a charge selection electrode mounted in said chamber, with said particle cloud produced between said plate and electrode; means for producing an electric field between said plate and electrode for attracting particles of one polarity to said electrode and particles of the opposite polarity to said plate, with the charge on said sheet attracting said particles of opposite polarity through the plate apertures to said sheet; a shutter mounted in said chamber between said plate and electrode and adjacent said plate; and means for moving said shutter between a closed position blocking said cloud of particles from said plate and an open position permitting movement of particles to said plate.
 2. A developer as defined in claim 1 including means for imparting oscillatory motion to said apertured plate in a plane substantially parallel to said receptor sheet.
 3. A developer as defined in claim 2 wherein said apertured image field plate is a wire screen.
 4. A developer as defined in claim 3 wherein said shutter comprises first and second planar elements positioned adjacent each other when in said first position and moved away from each other when in said second position. 